In a processing device of television signals it is necessary to process the radio frequency (RF) received signal before the signal is converted to a digital representation by the analog to digital converter (ADC). Such circuitry generally includes one or more amplifiers and one or more filters. Generally, a first RF gain amplifier is followed by a first out of band rejection filter, followed by a mixer for converting the signal to an intermediate frequency (IF), followed by a fixed frequency filter, such as a surface acoustic wave filter (SAW), followed by an IF gain amplifier. Those skilled in the art will recognize the need to amplify the signal prior to the SAW filter, as the latter will normally have a significant insertion loss, or attenuation, as a result. It is common to control the gain of RF or IF gain amplifiers independently using feed back gain control loops.
In an operation developed for the reception of analog television signals commonly used today in the reception of digital television signals, an analog closed-loop AGC is associated with the RF gain stage alone using an analog power detector. The detector will operate based on the total signal power. If there is a strong undesired signal present, the total power seen by the analog detector will also be higher and the detector output will drive the RF gain down, resulting in a lower desired channel power downstream. This will be sensed by an AGC generator, which will, in turn, request higher gain from the RF section. If an IF gain amplifier stage is used, the IF gain amplifier operates on a signal previously filtered by the SAW filter. In an analog television signal processing apparatus, some non-linear distortion was acceptable, whereas in a digital system, non-linear distortion can result in a level of signal corruption that is undecodable. In modern digital television signal receivers, it would be desirable to take advantage of the information available during digital signal processing to optimize the preliminary signal processing circuitry to ensure the best quality, or most receivable signal is delivered to the digital signal processing device.